A nonaqueous electrolyte secondary battery, especially a lithium ion secondary battery, has been used in household compact equipment such as a cell phone or a personal computer because of having high energy density. In recent years, application to automobiles has also been accelerated, in addition to the compact equipment.
The nonaqueous electrolyte secondary battery is a battery using an organic solvent-based electrolytic solution, and usually includes a positive electrode and a negative electrode, and also includes a separator disposed for the purpose of electrically insulating between these electrode plates. As a separator for a lithium ion secondary battery, for example, a microporous sheet made of a polyolefin-based resin is used.
When a short circuit occurs inside a battery, a shutdown function of the separator made of this microporous sheet leads to closing of pores of the separator so as to prevent movement of lithium ions at the short circuit part, and accordingly eliminating a battery function at the short circuit part. In such manner, the separator plays a role in maintaining safety of the lithium ion secondary battery. However, when the temperature of the battery exceeds, for example, 150° C. due to momentarily generated heat, the separator may drastically contract and the short circuit part of the positive electrode and the negative electrode may expand. In this case, the temperature of the battery may reach an abnormally overheated state of several hundred degrees centigrade or higher, and therefore, there is a problem in safety.
Patent Document 1 proposes technology of forming an inorganic oxide porous film including an inorganic oxide filler having insulation properties on a surface of a positive electrode, a negative electrode or a separator that constitutes a lithium ion secondary battery.